Autonomic Nervous System

The Autonomic Nervous System and Visceral Sensory Neurons

Autonomic Nervous System Concerned with the innervation and control of visceral organs, smooth muscles and glands Along with the endocrine system, its primary function is homeostasis of the internal environment The majority of the activities of the autonomic system do not impinge on consciousness The control exerted by the system is extremely rapid and widespread The visceral receptors include chemoreceptors, baroreceptors, and osmoreceptors. Ischemia or stretch can cause extreme pain Distributed both in the central and peripheral nervous system Like the somatic nervous system, it has sensory (afferent) & motor (efferent) neurons and interneurons

The afferent impulses originate in the visceral receptors, travel via afferent pathways to the CNS and terminate on the sensory neurons at different levels Like the somatic system, the cell bodies of the sensory neurons are located in the sensory ganglia SomaticAutonomic sensory ganglia

The efferent pathway is made up of preganglionic and postganglionic neurons The cell bodies of the preganglionic neurons are located in the brain and spinal cord. Their axons synapse with the postganglionic neurons whose cell bodies are located in the autonomic gangl ia

Comparison of Autonomic and Somatic Motor Systems Autonomic nervous system Autonomic nervous system – Chain of two motor neurons Preganglionic neuron Postganglionic neuron – Conduction is slower due to thinly or unmyelinated axons Pre-ganglionic Ganglion Post-ganglionic

Autonomic and Somatic Motor Systems

nicotinic receptors always cholinergic cholinergic or adrenergic muscarinic or adrenergic receptors

Visceral motor system Visceral motor system is different from somatic motor system in many respects Somatic motor system Autonomic motor system Effector Skeletal muscle Cardiac muscle, smooth muscle, glands Type of controlVoluntary Involuntary Neural pathwayOne motor neuron extends from the CNS to skeletal muscle Chain of two motor neurons: Preganglionic & Postganglionic neuron Action on effectorsAlways excitatoryMay be excitatory or inhibitory NeurotransmitterAcetylcholineAcetylcholine or norepinephrine Rate of conductionRapid due to myelinated axons Slower due to thinly myelinated or unmyelinated axons

Based on the anatomical, physiological and pharmacological characteristics, the autonomic nervous system is divided into:  Sympathetic: Activated during exercise, excitement, and emergencies. “fight, flight, or fright”  Parasympathetic: Concerned with conserving energy. “rest and digest”  Both divisions operate in conjunction with one another (have antagonistic control over the viscera) to maintain a stable internal environment

Parasympathetic Division brain & Forms craniosacral outflow: Issues from brain & S 2 -S 4 segments of spinal cord Ganglia are near or within the viscera (longer preganglionic fiber) Each preganglionic fiber usually synapses with four or five postganglionic neurons that pass to a single visceral effector Sympathetic Division Forms thoracolumbar outflow: Issues from T 1 - L 2 segments of spinal cord Ganglia are close to the CNS (longer postganglionic fiber) Each preganglionic fiber synapses with many postganglionic neurons that pass to many visceral effectors

Cranial Outflow Emerges from brain nuclei of the 3 rd,7 th, 9 th & 10 th cranial nerves, i Preganglionic neurons located in nuclei of the 3 rd,7 th, 9 th & 10 th cranial nerves, in the brain stem 3 rd,7 th, 9 th & 10 th cranial nerves Postganglionic fibers are carried by 3 rd,7 th, 9 th & 10 th cranial nerves and innervate organs of the head, neck, thorax, and abdomen Sacral Outflow Emerges from S 2 -S 4 Preganglionic neurons located in lateral horn of spinal gray matter Postganglionic fibers carried by pelvic splanchnic nerves to innervate organs of the pelvis and lower abdomen Parasympathetic Division

Parasympathetic Ganglia Multiple, small, located nearer the viscera Ganglia related to innervation of: Ganglia related to innervation of:  head & neck: (ciliary, otic, pterygopalatine & submandibular).  thoracic, abdominal & pelvic viscera.

T 1 -L 2 Thoracolumbar outflow: Emerges from T 1 -L 2 segments of spinal cord Preganglionic neurons located in the lateral gray horn. Sympathetic Division

Regardless of target, all begin same Preganglionic axons exit spinal cord through ventral root and enter spinal nerve Exit spinal nerve via communicating ramus Enter sympathetic trunk/chain where postganglionic neurons :- Has three options…

Options of preganglionic axons in sympathetic trunk 1.Synapse on postganglionic neuron in chain ganglion then return to spinal nerve and follow its branch to the skin 2.Ascend or descend within sympathetic trunk, synapse with a postganglionic neuron within a chain ganglion, and return to spinal nerve at that level and follow branches to skin 3.Enter sympathetic chain, pass through without synapsing, form a splanchnic nerve that passes toward thoracic or abdominal organs – These synapse in prevertebral ganglion in front of aorta – Postganglionic axons follow arteries to organs

Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Sympathetic Pathways to Periphery:Synapse in chain ganglia at same level or different level Figure 15.9 Preganglionic fibers run in the ventral roots of the spinal nerve Travel through the spinal nerve, and then join the sympathetic chain via the white rami communicans. (myelinated axons) (WRC) The postganglionic fibers enter back into the spinal nerve through grey rami communicans (GRC) (nonmyelinated axons)

Pass through ganglia and synapse in prevertebral ganglion

Sympathetic Ganglia Multiple, large in size Located nearer the central nervous system: Based on their relation to the vertebral column, they are grouped into:  Prevertebral  Paravertebral

Prevertebral Ganglia Unpaired, not segmentally arranged Located in abdomen, anterior to the vertebral column Main ganglia  Celiac  Superior mesenteric  Inferior mesenteric  Aorticorenal

Paravertebral Ganglia Consist of the right and left sympathetic chains or trunks. The chains lie next to the vertebral column throughout its length There is approximately one ganglion associated with each spinal cord segment, except in the cervical and the sacral regions. The chains end into a common ‘ganglion impar’ in front of coccyx

 T1 to L2 ventral rami are connected to the sympathetic chain via white rami communicantes, which carry preganglionic sympathetic fibers to the sympathetic chain  All the  All the ventral rami receive postganglionic sympathetic fibers from sympathetic chain by a gray ramus

The Role of the Adrenal Medulla in the Sympathetic Division Major organ of the sympathetic nervous system Secretes great quantities epinephrine (a little norepinephrine) Stimulated to secrete by preganglionic sympathetic fibers

The Adrenal Medulla

Neurotransmitter of the Autonomic Nervous System preganglionic axons  Acetylcholine for both divisions (cholinergic) postganglionic axons  Sympathetic: mostly norepinephrine  Parasympathetic: acetylcholine

Distribution of Autonomic Fibers Both divisions innervate mostly the same structures & operate in conjunction with one another (have antagonistic control over the viscus) to maintain a stable internal environment Some viscera do not possess dual control e.g. sweat glands, adrenal medulla, erector pili muscles and many blood vessels have only sympathetic innervation

Higher Control of the Autonomic Nervous System The hypothalamus has a controlling influence on the autonomic system and regulates balance between sympathetic and parasympathetic activity levels The hypothalamus integrates the autonomic and neuroendocrine systems to preserve body homeostasis